isTreeSymmetric.js

function get(object, prop, notFound = undefined) {
  if (object) {
    if (prop in object) {
      return object[prop];
    }
  }
  return notFound;
}

function depthFirst(tree, order, isNotSymmetrical, callback) {
  const value = get(tree, 'value');
  const left = get(tree, 'left');
  const right = get(tree, 'right');
  
  if (right && !left) {
    // console.log('is not symmetrical')
    isNotSymmetrical = true;
    // callback(value, true)
  }
  
  if (left && !right) {
    isNotSymmetrical = true;
  }
  
  order === "pre" && callback(value, isNotSymmetrical);
  left && depthFirst(left, order, isNotSymmetrical, callback);
  order === "in" && callback(value, isNotSymmetrical);
  right && depthFirst(right, order, isNotSymmetrical, callback);
  order === "post" && callback(value, isNotSymmetrical);
}

function isTreeSymmetric(t) {
  let isNotSymmetrical = false;
  const _in = [];
  depthFirst(t, 'in', isNotSymmetrical, (val, notSymm = false) => {
    if (notSymm) {
      isNotSymmetrical = notSymm;
    }
    _in.push(val);
  });
  
  if (isNotSymmetrical) {
    // return false;
  }
  
  // in
  if (_in.length % 2 === 1) {
    const arrLen = (_in.length - 1) / 2;
    const left = _in.splice(0, arrLen);
    _in.splice(0,1);
    const right = _in.reverse();
    
    return left.every((int, index) => {
      return int === right[index];
    })
  } 
  
  return false;  
}